: The endosomal cysteine protease, cathepsin S, has a key role in MHC class II (MHCII)-dependent immunity through its role in degradation of the MHCII-associated chaperone, the invariant chain (Ii). But studies of cathepsin S -/- mice also reveal unexpected findings. Cathepsin S deficiency predominantly affects Thl dependent immunity. IgE responses and Th2-driven pulmonary inflammation, while dependent on cysteine proteases, are normal or increased in cathepsin S -/- mice. Indeed baseline IgE levels and lung mRNA of proinflammatory cytokines in these mice are elevated. Surprisingly, high IgE levels and cytokines are mast cell-dependent. These observations imply additional proteases, and perhaps additional antigen presenting cells (APC), are important to MHCII function and that protease dysregulation can skew, not just block, MHCII-dependent immune responses. Experiments are directed toward understanding how proteases can effect polarization of T and B cell development relevant to lung disease. Mechanisms of peptide loading in cathepsin S-deficient APC will be defined. Mice deficient in the Ii-degrading APC protease cathepsin F have been recently generated and will be used to test the hypothesis that cathepsin F rescues MHC class II peptide display in myeloid APCs, thereby favoring Th2 polarization. Selective protease inhibitors will be used to determine if a set of protease activities in human APC are required for Th2 cytokine production by allergen stimulated T cells. The mechanisms by which mast cells promote APC-dependent IgE production will be explored by reconstitution of mast cell deficient mice (Wsh) with mast cells cultured from S-/- or cytokine deficient mice. The overall goal of this application is to understand the molecular basis for protease dependent polarization of T cell and B cell immune responses and to learn how to exploit this process to ameliorate CD4+ T cell driven disorders such as autoimmunity and asthma.